The World in a Tree Hole

The World in a Tree Hole

Against the white plastic, a black speck is silhouetted: a helodid beetle climbing away from the brownish leaf-filled water.

Why is this beetle here, in a plastic container covered with a screen lid, instead of in the hole-in-a-tree it calls its home?

Paradise transferred its natural dwelling—water, leaves, and all—from the forest to a walk-in cooler. Dozens of these microcosms fill the shelves. Paradise, a graduate student in biology, will use them to determine the relationship between the helodid beetle and another insect, a kind of biting midge.

Earlier that afternoon, in an oak woods a mile north of his lab, Paradise reached into the hole at the base of a tree and pulled out dripping leaves one by one, inspecting each for critters. Finally, when all the surface leaves were removed, the two of us looked into the still liquid.

"Back in the early part of the century, when all the local forests were clearcut, many of the leftover stumps had the ability to re-sprout," he explained. "And so new trunks would come up joined to the old stumps. Then the stumps rotted out to form basins that would fill with water, forming treeholes." These basal treeholes are much easier to study than holes higher on a tree, because they are larger and have higher densities of organisms. How much larger? The higher ones (the size of a knot-hole) usually hold only about 100 milliliters of water, while the basal ones can hold anywhere from one to 30 liters.

Paradise poked in a stick. It went far in, maybe ten inches down. He took out the stick and discovered something on the end: the intricate skeleton of a leaf. Helodid beetles had eaten all of it but the veins, left for other organisms to munch on. When Paradise placed it on the surface of the water, the leaf bent and folded its way back to the bottom. He pointed out another sign of the beetle's presence: translucent specks on the water's surface, the molted pieces of the beetle's outer skin.

Paradise, with his adviser, biologist Bill Dunson, has been studying the treeholes of Centre County, Pennsylvania, for more than three years. He wants to understand what goes on inside so that he can predict how treehole communities will be affected by changes in the environment.

"One thing that's nice about studying treeholes is their small size," says Paradise. "These systems contain defined boundaries and a defined set of organisms." Compared to other ecosystems, they are easily manipulated indoors, with little divergence from the natural environment. The microcosm size equals that of the natural treehole.

Paradise has tested the effects of water volume, the concentration of dissolved organic carbon or sodium in the water, and the amount of leaf litter in the hole on the diversity and density of insect species living in a treehole ecosystem. For instance, treeholes with more water had higher densities of both mosquitoes and helodid beetles. Those with high levels of dissolved organic carbon, however, kept the beetles' population high while reducing the mosquitoes'. High sodium concentrations, on the other hand, resulted in larger female mosquitoes and fewer helodid beetles.

Recently Paradise has found that in the presence of helodid beetles, the size of adult midges increases. Yet midges have a lower survival rate than beetles do in the slightly acidic, low pH water of these tree-holes, says Paradise. "If you put these two organisms together at low pH, you might hypothesize that the facilitation of the beetles overcomes the adverse affects of pH, and the midges do fine." The leaf-shredding beetles might make food more available to the midges, which eat smaller particles, allowing the midges to survive better under acidic conditions.

On the other hand, the beetles might be aiding the midges indirectly by providing a food source for protozoa, microorganisms that might be eating the skeletonized leaves. In turn, the midges could be feeding on these protozoa. Paradise is currently studying this relationship, which ecologists call commensalism, since the midges benefit and there is neither a positive nor negative effect on the beetles.

It's in order to test this hypothesis—that the commensalistic relationship with the beetles overrides the midges' susceptibility to low pH—that Paradise keeps the 48 treehole microcosms in his walk-in cooler. The systems are contained in one-liter plastic containers half full of water and covered with screen lids. A strip of screen placed on the inner walls of each allows the beetles to climb out of the water to gather oxygen.

"The commensalistic relationship may be widespread," Paradise says. If so, the little leaf-shredding helodid beetle could be quite important in the treehole world.